Physiological parameter monitoring system and sensor assembly for same

ABSTRACT

A detachable sensor assembly for use in monitoring a physiological parameter of a subject includes an information containing circuit and a timer. The sensor assembly stores time varying information in the information containing circuit. The information may include cumulative use times for the detachable sensor assembly and/or for components in the detachable sensor assembly.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing dates of U.S. patentapplication Ser. No. 60/330,425 filed on 22 Oct. 2001 and entitledPHYSIOLOGICAL PARAMETER MONITORING SYSTEM AND SMART SENSOR ASSEMBLY FORSAME and application No. 60/347,870 filed on 15 Jan., 2002 and entitledSMART SENSOR ASSEMBLY FOR MONITORING PHYSIOLOGICAL PARAMETERS, both ofwhich are hereby incorporated herein by reference.

TECHNICAL FIELD

This application relates to medical devices, and in particular tosystems for monitoring one or more physiological parameters of asubject. The invention relates to such systems which include one or moredetachable sensors. The sensors may include pulse oximetry sensors andmay be connected to deliver signals to a monitoring device by way of acord comprising one or more signal transmission lines.

BACKGROUND

Modern medical devices have sensors for measuring various physiologicalparameters of patients. The inventor has determined that there is a needfor automated systems which facilitate the management of such sensors.

SUMMARY OF THE INVENTION

This invention provides sensor assemblies which incorporate circuitswhich store information relating to the sensor assemblies. Sensorassemblies according to some aspects of the invention include processorsfor processing data.

One aspect of the invention provides detachable sensor assemblies forsupplying signals to devices for monitoring physiological parameters ofsubjects. The detachable sensor assemblies comprise: a sensor; aninformation containing circuit; a timer; and, a connector comprising oneor more signal conductors connected to carry information from the sensorand information from the information containing circuit to a device formonitoring a physiological parameter. The information containing circuitis configured to store time-varying information in response to timingsignals from the timer and to transmit the time varying information byway of the connector. In sensor assemblies according to specificembodiments of the invention the time varying information comprises oneor more of:

-   a cumulative time during which power has been applied to the sensor;-   a cumulative time during which an active component in the sensor has    been operating;-   a cumulative time during which the sensor has been acquiring data    from a subject;-   a cumulative time since a sensor reset;-   a cumulative time since a sensor error;-   a cumulative time since the sensor was calibrated; and,-   a cumulative time since an interruption in a signal detected by the    sensor.

Another aspect of the invention provides a system for determining avalue of a physiological parameter comprising a detachable sensorassembly as described above connected to a monitoring device. Themonitoring device may be configured to: retrieve a first instance of thetime varying value from the information containing circuit at a firsttime; store the first instance of the time varying value in a memory;retrieve a second instance of the time varying value from theinformation containing circuit at a second time later than the firsttime; and, compare the first and second instances of the time varyingvalue to a difference between the first and second times. The timevarying value may comprise a cumulative use time for a component in thedetachable sensor assembly and the monitoring device may be configuredto energize the component at or before the first time.

Another aspect of the invention provides apparatus for monitoring aphysiological parameter of a subject. The apparatus comprises amonitoring device comprising stored information identifying one or moreacceptable sensor combinations, each of the acceptable sensorcombinations comprising a plurality of sensor locations required for thedetermination of a physiological parameter; and, a plurality of sensorsdetachably connected to the monitoring device. Each of the sensors isintended for application to a different location on a subject's anatomy.Each of the sensors comprises a circuit containing stored informationindicating the intended location for the sensor. The monitoring devicecomprises a processor connected to retrieve the stored information fromeach of the plurality of sensors, and to determine from the retrievedstored information whether the plurality of sensors includes all sensorsof at least one of the acceptable sensor combinations.

A further aspect of the invention provides a method for operating adetachable sensor assembly comprising a sensor for supplying a signal toa monitoring device for monitoring a physiological parameter of asubject. The method comprises determining in the detachable sensorassembly a time varying value; storing the time varying value in a datastore on the detachable sensor assembly a time varying value; and,transmitting the time varying value to a monitoring device detachablyconnected to the detachable sensor assembly.

Further aspects of this invention and features of specific embodimentsof this invention are described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In drawings which illustrate non-limiting embodiments of the invention:

FIG. 1 is an isometric view of a Y-type connector according to theinvention;

FIG. 2 is an isometric view of a sensor which may be connected to theY-type connector of FIG. 1;

FIG. 3 is a circuit diagram of an assembly comprising the connector ofFIG. 1 connected to a pair of sensors similar to those shown in FIG. 2;and,

FIG. 4 is a schematic view of a system including a monitoring deviceaccording to the invention and a sensor according to the invention.

DESCRIPTION

Throughout the following description, specific details are set forth inorder to provide a more thorough understanding of the invention.However, the invention may be practiced without these particulars. Inother instances, well known elements have not been shown or described indetail to avoid unnecessarily obscuring the invention. Accordingly, thespecification and drawings are to be regarded in an illustrative, ratherthan a restrictive, sense.

The invention relates to a removable sensor which includes a circuitcontaining information about the sensor. In preferred embodiment thecircuit includes a memory capable of being updated with new information.The circuit may comprise a single-chip microcontroller, for example, amodel PIC12CE518/9 available from Microchip Technology Inc. of Arizona.The circuit may comprise a memory device, such as a serial electricallyerasable PROM memory chip. The memory device could comprise, forexample, a model 24AA00/LC00 serial electrically erasable PROM alsoavailable from Microchip Technology Incorporated. In some embodimentsthe sensor may be a disposable sensor or a sensor having a limited lifespan.

The information contained in the circuit may include information aboutthe sensor itself. For example, the information may include:

-   information about the manufacturer of the sensor;-   information about the model of the sensor;-   information about the type of the sensor;-   a unique serial number for the sensor;-   information about the date of manufacture of the sensor;-   information about the intended use of the sensor (e.g. for a    pulse-oximetry sensor, information regarding a location on a    subject's body at which the sensor is designed to be placed);-   calibration information for the sensor;-   information about an operational status of the sensor; or,-   information about faults in the sensor.

The information may include information regarding uses of the sensorsuch as:

-   an indication as to whether the sensor has been used;-   an indication as to when the sensor was last serviced or calibrated;-   identification of a subject in association with which the sensor has    been used (for sensors which are intended for use only with a    specific subject and should be thrown away or refurbished before use    on another subject);-   information about when the sensor was used, or used first;-   information about a cumulative length of time during which the    sensor was used; or,-   information about a number of times that the sensor has been used.

The invention also relates to a system which includes a sensor accordingto the invention connected to deliver a signal to a monitoring device.The monitoring device may obtain information from the circuit and takeactions in response to the received information. Where the sensor is ofa disposable type, the information retrieved may relate to previous usesof the sensor. If the information indicates that the sensor's servicelife is over then the action taken may be to reject the signal from thesensor. If the information indicates that the sensor's service life isnot over then, during the use of the sensor the monitoring device maysend new information regarding the use of the sensor to be stored by thecircuit. The new information may comprise information of one or more ofthe types described above.

The information retrieved by the monitoring device may also relate tothe sensor itself. In cases where the monitoring device requires signalsfrom multiple sensors in order to determine a value for a physiologicalparameter then the monitoring device may use information received fromthe circuits on each of the sensors to verify that an appropriatecombination of sensors has been connected for the test being conducted.One situation where this capability is useful is in making differentialpulse transit time based blood pressure measurements. A system formaking such measurements is described in commonly owned internationalapplication No. PCT/No. PCT/CA00/01552 filed 22 Dec., 2000 and entitledCONTINUOUS BLOOD PRESSURE MONITORING METHOD AND APPARATUS, which ishereby incorporated by reference herein.

Differential pulse transit time measurements involve detecting thearrival of a pulse wave at different locations on a subject's body.There is a correlation between the difference between the times at whichthe pulse wave is detected at the two locations and the subject's bloodpressure. Differential pulse transit time based blood pressuremeasurements generally require sensors to be located at differentlocations on a subject's body. The two sensors may, for example, each belocated at different locations selected from the subject's:

-   forehead;-   ear lobe;-   fingertip;-   toe;-   femoral artery; etc.    Typically sensors designed for each of these locations have a    different configuration. The calibration of the monitoring device    will depend upon which pair of locations is chosen for the sensors.

Where the sensors being used are equipped with information-containingcircuits according to this invention then the monitoring device can putthe information in the circuits on the sensors to use in various ways.The monitoring device may first check to ensure that an appropriatecombination of sensors has been selected. For example, if the monitoringdevices retrieves from both sensors indicating that the sensors are bothof a type adapted for use at the same location then the monitoringdevice may generate a warning indication or may refuse to operate.

In some cases a monitoring device may be calibrated to determine theblood pressure of a particular subject for two or more different pairsof sensor locations. For example, the monitoring device may maintain afirst set of calibration information which correlates pulse signalsdetected at the subject's ear lobe and fingertip to the subject's bloodpressure and a second set of calibration information which correlatespulse signals detected at the subject's earlobe and toe to the subject'sblood pressure. By retrieving information from each of the sensorsregarding the location at which the sensor is designed to be used themonitoring device can verify that it has calibration information forthat pair of sensor locations and can select for use the calibrationinformation appropriate for the pair of sensors being used.

While the invention also has application to other types of sensors ithas particular application to sensors of the type used for pulseoximetry. Pulse oximetry sensors are commonly used to measure the oxygensaturation of a subject's blood and also to measure a subject's pulserate.

A typical pulse sensor typically comprises one or more (most typicallytwo or more) light sources. The light sources may compriselight-emitting diodes, and a light detector, such as a phototransistoror photodiode. The photo-transistor detects light which is either passedthrough a fold of the subject's skin or has been reflected from thesubject's skin. A signal detected by the photo-transistor (or otherlight detector) is returned to an analysis circuit in a monitoringdevice. The monitoring device determines from the signal values ofphysiological parameters such as pulse rate and blood oxygen saturation.

In monitoring systems which makes use of multiple sensors there are morepoints at which faults can occur. Diagnosing such faults is moredifficult in a device which has multiple sensors than it is in a device,such as a simple pulse oximetry system, which has only a single sensor.The information circuit of the invention may comprise circuitry fordetecting common faults in the sensor with which it is associated. Thisprovides useful capabilities for pinpointing defective sensors.

The sensor may comprise a timer. The timer may be integrated with theinformation containing circuit or may be separate. The timer may track aduration in which the sensor is in use. Where the information containingcircuit comprises a microprocessor the timer may comprise a clockassociated with the microprocessor. In the alternative, the timer maycomprise a separate timing circuit. The information containing circuitmay comprise a memory location 19A storing a current timer value. Thecurrent timer value is updated in response to signals from the timer.

Memory location 19A may be in a persistent memory. This is not necessaryfor all applications.

The timer may track the duration of events of significance to thesensor. For example, the timer may track one or more of:

-   a cumulative time during which the sensor has been “on”;-   a cumulative time during which an active component in the sensor has    been operating;-   a cumulative time during which the sensor has been acquiring data    from a subject; and,-   a cumulative time since an event, such as a sensor reset, a sensor    error, the sensor was calibrated, an interruption in a signal    detected by the sensor, an unusual signal condition, or the like.

As shown in FIG. 2, a sensor assembly 10 according to one embodiment ofthe invention comprises a sensor 12, a cord 14 and a connector 16 forconnecting the sensor directly or indirectly to a monitoring device. Aninformation circuit 18 is incorporated into the sensor. In theillustrated embodiment, information circuit 18 comprises a chip builtinto connector 16. In the illustrated embodiment, information circuit 18comprises a timer 19.

When sensor assembly 10 is connected to a monitoring device 20 by way ofconnector 16 (See FIG. 4) the information circuit is able to communicatewith the monitoring device. Communication between the informationcircuit 18 and the monitoring device may be carried, for example, byconductors in connector 16. The conductors may comprise optical fibers,electrical conductors or other media capable of carrying informationbetween information circuit 18 and a monitoring device.

Sensor assembly 10 may be connected directly to monitoring device 20 (ifmonitoring device 20 has connectors capable of mating with connector 16)or may comprise a suitable extension cable extending between connector16 and monitoring device 20. Especially where sensor assembly 10 is of atype which is intended to be disposable, it can be desirable to use anextension cable so that the segment 14 of cable in the disposableportion of sensor assembly 10 is relatively short.

FIG. 1 shows an example of a possible extension and adapter cable 22.Cable 22 has a first connector 24 which mates with a connector onmonitoring device 20. A cable 26 which has branches 26A and 26B carriessignal conductors to second connectors 28A and 28B (collectivelyconnectors 28). Connectors 28 are configured to mate with connectors 16on sensor assemblies 10. An extension and adapter cable for use insystems according to the invention may have one, two, three or morebranches each bearing a connector 28.

FIG. 3 is a circuit diagram showing data and power connectors in asystem according to the invention which includes a pair of sensorassemblies 10, a Y-type connector cable 22 and a monitoring device 20.In this example embodiment of the invention each sensor 10 comprises alight emitter 30 (which may comprise light-emitting diodes (LEDs)) and aphototransistor or photodiode 32. Conductors 34-1 through 34-12B carrysignals to and from monitoring device 20 and carry electrical power tosensor assemblies 10. Conductors 34-2 and 34-3 carry a signal from afirst one 10A of the sensor assemblies to monitoring device 20.Conductors 34-8 and 34-9 carry signals from a second one 10B of thesensor assemblies to monitoring device 10.

In this embodiment each information circuit 18 comprises a programmedmicrocontroller which has an integrated program memory, an integratedtimer 19 and an integrated non-volatile memory capable of storinginformation regarding the sensor assembly 10 and/or the use history ofthe sensor assembly 10. Monitoring device 20 maintains bidirectionalcommunication with the information circuit 18 associated with firstsensor assembly 10A through conductor 34-6A and 34-6B. Monitoring device20 maintains bidirectional communication with the information circuit 18associated with second sensor assembly 10B through conductors 34-12A and34-12B. In the illustrated embodiment, information and commands forcircuit 18 are carried serially. In other embodiments of the inventionone or more separate data lines may be provided for communication ineach direction. Information may also be exchanged between an informationcircuit 18 and a monitoring device 20 in parallel fashion.

It is a matter of design choice as to whether circuit 18 is configuredto send only specific information to monitoring device 18 in response torequests for that specific information from monitoring device 20 orwhether in response to a request for information from monitoring device20, information circuit 18 automatically sends multiple items ofinformation to monitoring device 20. In the latter case, monitoringdevice 20 can sort through the information retrieved from informationcircuit 18 to obtain any specific item(s) of information that itrequires. Circuit 18 could also, or in the alternative, make periodictransmissions of information to monitoring device 20.

Information circuit 18 may be connected (or selectively connectable) totest points in the associated sensor 12 so that information circuit 18can obtain information about the normal operation of sensor 12 and/orinformation that may be relevant to the existence of faults in sensor12. Information circuit 18 may perform diagnostic checks of sensor 12and/or provide to monitoring device 20 information useful for conductingdiagnostic checks of sensor 12.

As shown schematically in FIG. 4, monitoring device 20 may comprise anumber of functional aspects which interact with information circuits 18in connected sensor assemblies 10. These functional aspects may beprovided in software instructions executed by a programmed dataprocessor, in hardware, or in a combination of hardware and software.The functional aspects are not necessarily separate and distinct fromone another in that components for providing one function may also beused by components for providing other functions.

In the embodiment illustrated in FIG. 4, monitoring device 20 comprisesa sensor validation function 40. Sensor validation function 40 retrievesinformation regarding previous usage of sensor assemblies 10 frominformation circuits 18 and determines if the service life of any one ofsensors has been exceeded. This determination may be based upon:

-   whether the information from information circuit 18 indicates that    the sensor assembly has been previously been used—for example, for    sensor assemblies designed for single use;-   whether a cumulative service life exceeds a threshold value (the    threshold value may be included in information stored in information    circuit 18 and forwarded to monitoring device 20, the comparison may    be performed at information circuit 18 and/or the threshold value    may be stored in monitoring device 20)(the cumulative service life    may comprise one or more of, or a combination of—a time that the    sensor has been energized, a time that an active component of the    sensor has been energized, a time since the sensor was first used, a    time during which the sensor has detected a signal from a subject,    and the like)—for example, for sensor assemblies that tend to    exhibit degraded performance as they are used;-   whether a lifespan has been exceeded from the time that the sensor    assembly 10 was first used or manufactured—for example, for sensor    assemblies 10 which tend to exhibit performance which degrades with    time whether or not the sensor assembly is being used;-   whether an excessive time has passed since the sensor assembly 10    was last calibrated;-   and so on.

Monitoring device 20 includes a sensor testing function 42. Sensortesting function 42 tests sensor assemblies 10 for faults by eithertesting directly signals received from sensor assemblies 10, retrievinginformation about fault conditions from information circuits 18 or both.Where sensor assemblies 10 include a timer 19, sensor testing function42 may periodically retrieve a value from memory location 19A to verifythat timer 19 is operating correctly. For example, sensor testingfunction 42 may store a value previously retrieved from memory location19A. When sensor testing function 42 receives a new value from memorylocation 19A it can compare a difference between the new and previousvalues to a length of time between acquisition of the new and previousvalues to verify that timer 19 is functioning properly.

Where memory location 19A comprises a value which represents an amountof time during which a certain component of sensor assembly 10 has beenenergized then sensor testing function 42 may cause monitoring system 20to energize the component for a period of time and compare the valuesstored in memory location 19A before and after the component wasenergized to verify that timer 19 is accurately tracking the time duringwhich the component is energized.

Monitoring device 20 includes physiological parameter estimationcomponent 44 which receives signals from one or more sensor assemblies10 and derives estimates of the physiological parameters of a subjectfrom those signals. By way of example only, component 44 may determine asubject's pulse rate, blood oxygen saturation, and/or blood pressure.

Monitoring device 20 includes a clock/calendar component 46 whichmaintains a record of the current time and date. A writing component 48sends information to an information circuit 18 for recording in a memoryassociated with information circuit 18. The information may include thedate and time that the sensor assembly 10 associated with theinformation circuit 18 is being used and the duration of the use.

Monitoring device 20 includes a sensor-type checking component 49.Sensor type checking component 49 obtains information from informationcircuits 18 regarding the types of sensors connected to monitoringdevice 20 and verifies that those types of sensor match a function to beperformed by physiological parameter estimation component 44. Sensortype-checking component may pass information regarding the types ofconnected sensors to physiological parameter estimation component 44. Byway of example only, physiological parameter estimation component 44 mayuse this information to configure itself to estimate a physiologicalparameter for a subject based on signals expected to be received fromsensors identified by sensor type-checking component 49.

For example sensor type checking component 49 may determine that theparticular sensors connected to monitoring device 20 are for use by aparticular known subject. Further, example sensor type checkingcomponent 49 may determine that the sensors in question are an earlobesensor and a fingertip sensor. Physiological parameter estimationcomponent 44 configures itself in response to receiving this informationby selecting calibration information for the known subject correspondingto the use of an earlobe and fingertip sensor.

Monitoring device 20 may include a display which displays informationretrieved from information circuits 18. The display may, for example,display the types of sensors detected and the name of the subjectassociated with the sensors. An operator can use the displayedinformation to check that a procedure is being performed properly on thecorrect subject.

EXAMPLE

In an example embodiment of the invention, a sensor assembly 10comprises a pulse oximetry sensor. A clinician wishes to use a sensorassembly 10 to monitor some physiological parameter of a subject. Theclinician connects the sensor assembly 10 to a monitoring device 20.Sensor assembly 10 comprises an information circuit 18. Upon sensorassembly 10 being made operational, (for example, upon connection tomonitoring device 20, upon monitoring device 20 being turned on, or uponmonitoring device 20 energizing sensor assembly 10) monitoring device 20receives information from information circuit 18. In this example, theinformation comprises information regarding the type and model number ofthe sensor, the date of manufacture, the cumulative use time of thesensor, and the cumulative “on” time of a light emitting device in thesensor. In this example, information circuit 18 comprises amicroprocessor which, upon being powered up performs a sensorinitialization routine which includes instructions which, when executedby the microprocessor, cause the microprocessor to retrieve from astorage location on sensor assembly 10 and forward to a connectedmonitoring device 20 the information.

Monitoring device 20 performs a monitor initialization routine uponconnection of sensor assembly 10. The monitor initialization routine maybe performed periodically after sensor assembly 10 has been connected.The monitor initialization routine performs a number of checks on theinformation. In this example, the monitor initialization routinecompletes successfully if:

-   the date of manufacture of the sensor is longer before a current    date maintained in the monitoring system than a threshold time;-   the cumulative use time of the sensor does not exceed a threshold    amount;-   the cumulative “on” time of a light emitting device in the sensor    does not exceed a threshold amount;-   the information is consistent with information previously recorded    in monitoring device 20 and associated with the unique serial number    for the sensor.    If the monitor initialization routine does not complete successfully    then monitoring device 20 signals an error.

While sensor assembly 10 is operational (for example, while monitoringdevice 20 is applying power to sensor assembly 10) information circuit18 periodically updates and stores in a memory location of sensorassembly 10 the cumulative use time. Information circuit 18 alsomonitors whether or not the light emitting device is energized andstores in another memory location the cumulative “on” time of the lightemitting device. Information circuit 18 periodically sends informationabout the cumulative use time to monitoring device 20.

Monitoring device 20 uses the information about the cumulative use timeto check the operation of the timer function of sensor assembly 10 asdescribed above. If the check indicates that the timer function is notoperating properly then monitoring device 20 signals an error.

During use information circuit 18 monitors various aspects of theoperation of the sensor. Information circuit 18 may send statusinformation periodically back to monitoring device 20. If the statusinformation indicates a sensor failure, or if monitoring device 20 failsto receive the status information for a time which is longer than athreshold time then monitoring device 20 signals an error.

Sensor assembly 10 acquires a signal from a subject. The signal passesto monitoring device 20 for analysis.

As will be apparent to those skilled in the art in tie light of theforegoing disclosure, many alterations and modifications are possible inthe practice of this invention without departing from the spirit orscope thereof. For example, a single sensor assembly may comprisemultiple sensors. Where this is the case a single information circuitmay be provided for the sensor assembly. The different sensors andinformation circuits connected to a monitoring device 20 may sharecommon power circuits, ground connections and the like in ways which areconsistent with maintaining acceptable signal quality. The foregoingdescription includes many features which may be used in the combinationsdescribed above, in other combinations or individually.

1. A system for determining a value of a physiological parameter, thesystem comprising a detachable sensor assembly for supplying a signal toa device for monitoring a physiological parameter of a subject, thedetachable sensor assembly comprising: a sensor; an informationcontaining circuit; a timer; and, a connector comprising one or moresignal conductors connected to carry information from the sensor andinformation from the information containing circuit to a device formonitoring a physiological parameter; wherein the information containingcircuit is configured to store time-varying information in response totiming signals from the timer and to transmit the time varyinginformation by way of the connector; wherein the time-varyinginformation comprises a cumulative use time for a component comprising alight-emitting element in the detachable sensor assembly the sensorassembly connected by way of the connector to a monitoring device;wherein the monitoring device is configured to: retrieve a firstinstance of a time-varying value from the information containing circuitat a first time; store the first instance of the time-varying value in amemory; retrieve a second instance of the time-varying value from theinformation containing circuit at a second time later than the firsttime; compare the first and second instances of the time-varying valueto a difference between the first and second times; energize thecomponent at or before the first time; retrieve from the informationcontaining circuit information identifying a date of manufacturing thedetachable sensor assembly; compare the date of manufacture to a currentdate maintained in the monitoring device; and, signal an error if thedate of manufacture of the sensor assembly is longer before the currentdate than a threshold time; retrieve from the information containingcircuit information identifying a cumulative use time of the detachablesensor assembly; signal an error if the cumulative use time exceeds athreshold; retrieve from the information containing circuit informationidentifying a cumulative on time of the component in the detachablesensor assembly; signal an error if the cumulative on time exceeds athreshold; retrieve information from the information containing circuit;compare the retrieved information to information stored in themonitoring device which was previously retrieved from the informationcontaining circuit; and, signal an error if the retrieved information isnot consistent with the stored information; wherein the monitoringdevice comprises a data store holding stored information identifying oneor more acceptable sensor combinations, each of the acceptable sensorcombinations comprising a plurality of sensor locations required for thedetermination of a physiological parameter; and, a plurality of sensorsincluding said sensor, each detachably connected to the monitoringdevice, each of the sensors intended for application to a differentlocation on a subject's anatomy, each of the sensors comprising acircuit containing stored information indicating the intended locationfor the sensor; wherein the monitoring device comprises a processorconnected to retrieve the stored information from each of the pluralityof sensors, and to determine from the retrieved stored informationwhether the plurality of sensors includes all sensors of at least one ofthe acceptable sensor combinations.
 2. A system according to claim 1wherein the monitoring device comprises stored calibration informationcorresponding to each of the acceptable sensor combinations and theprocessor is configured to determine a value for the physiologicalparameter by applying the calibration information corresponding to theidentified acceptable sensor combination to process signals from aplurality of the plurality of sensors corresponding to the identifiedacceptable sensor combination.
 3. A system according to claim 2 whereinthe physiological parameter comprises a blood pressure.
 4. A systemaccording to claim 3 wherein the sensors each comprise a pulse oximetrysensor.
 5. Apparatus for monitoring a physiological parameter of asubject, the apparatus comprising: a monitoring device comprising storedinformation identifying one or more acceptable sensor combinations, eachof the acceptable sensor combinations comprising a plurality of sensorlocations required for the determination of a physiological parameter;and, a plurality of sensors detachably connected to the monitoringdevice, each of the sensors intended for application to a differentlocation on a subject's anatomy, each of the sensors comprising acircuit containing stored information indicating the intended locationfor the sensor; wherein the monitoring device comprises a processorconnected to retrieve the stored information from each of the pluralityof sensors, and to determine from the retrieved stored informationwhether the plurality of sensors includes all sensors of at least one ofthe acceptable sensor combinations.
 6. The apparatus of claim 5 whereinthe monitoring device comprises stored calibration informationcorresponding to each of the acceptable sensor combinations and theprocessor is configured to determine a value for the physiologicalparameter by applying the calibration information corresponding to theidentified acceptable sensor combination to process signals from aplurality of the plurality of sensors corresponding to the identifiedacceptable sensor combination.
 7. The apparatus of claim 6 wherein thephysiological parameter comprises a blood pressure.
 8. The apparatus ofclaim 7 wherein the sensors each comprise a pulse oximetry sensor.
 9. Asystem for determining a value of a physiological parameter, the systemcomprising a detachable sensor assembly for supplying a signal to adevice for monitoring a physiological parameter of a subject, thedetachable sensor assembly comprising: a sensor; an informationcontaining circuit; a timer; and, a connector comprising one or moresignal conductors connected to carry information from the sensor andinformation from the information containing circuit to a device formonitoring a physiological parameter; wherein the information containingcircuit is configured to store time-varying information in response totiming signals from the timer and to transmit the time varyinginformation by way of the connector; wherein the monitoring devicecomprises a data store holding stored information identifying one ormore acceptable sensor combinations, each of the acceptable sensorcombinations comprising a plurality of sensor locations required for thedetermination of a physiological parameter; and, a plurality of sensorsincluding said sensor, each detachably connected to the monitoringdevice, each of the sensors intended for application to a differentlocation on a subject's anatomy, each of the sensors comprising acircuit containing stored information indicating the intended locationfor the sensor; wherein the monitoring device comprises a processorconnected to retrieve the stored information from each of the pluralityof sensors, and to determine from the retrieved stored informationwhether the plurality of sensors includes all sensors of at least one ofthe acceptable sensor combinations.
 10. A system according to claim 9wherein the information containing circuit comprises a microprocessor.11. A system according to claim 10 wherein the timer comprises a clockof the microprocessor.
 12. A system according to claim 9 wherein thesensor comprises a pulse oximetry sensor.
 13. A system according toclaim 9 wherein the information containing circuit comprises storedinformation comprising one or more of: information identifying amanufacturer of the detachable sensor assembly; information identifyinga model of the detachable sensor assembly; information identifying atype of the detachable sensor assembly; a unique serial number for thedetachable sensor assembly; information identifying a date ofmanufacture of the detachable sensor assembly; information identifying alocation on a subject's body at which the sensor is intended to beplaced; and, calibration information for the sensor.
 14. A systemaccording to claim 9 wherein the information containing circuit isconfigured to transmit the time varying information in response to arequest signal received by way of the connector.
 15. A system accordingto claim 9 wherein the information containing circuit is configured totransmit the time varying information periodically in response to atiming signal from the timer.
 16. A system according to claim 9 whereinthe information containing circuit comprises a non-volatile memory. 17.A system according to claim 9 wherein the monitoring device isconfigured to: retrieve a first instance of the time varying value fromthe information containing circuit at a first time; store the firstinstance of the time varying value in a memory; retrieve a secondinstance of the time varying value from the information containingcircuit at a second time later than the first time; and, compare thefirst and second instances of the time varying value to a differencebetween the first and second times.
 18. A system according to claim 17wherein the time varying value comprises a cumulative use time for acomponent in the detachable sensor assembly and the monitoring device isconfigured to energize the component at or before the first time.
 19. Asystem according to claim 18 wherein the monitoring device is configuredto: retrieve from the information containing circuit informationidentifying a date of manufacturing the detachable sensor assembly;compare the date of manufacture to a current date maintained in themonitoring device; and, signal an error if the date of manufacture ofthe sensor assembly is longer before the current date than a thresholdtime.
 20. A system according to claim 19 wherein the monitoring deviceis configured to: retrieve from the information containing circuitinformation identifying a cumulative use time of the detachable sensorassembly; and signal an error if the cumulative use time exceeds athreshold.
 21. A system according to claim 20 wherein the monitoringdevice is configured to: retrieve from the information containingcircuit information identifying a cumulative on time of a component inthe detachable sensor assembly; and, signal an error if the cumulativeon time exceeds a threshold.
 22. A system according to claim 21 whereinthe component comprises a light emitting element.